INTRA‐ANNUAL VARIATION IN RIVER–RESERVOIR INTERFACE FISH ASSEMBLAGES: IMPLICATIONS FOR FISH CONSERVATION AND MANAGEMENT IN REGULATED RIVERS

While much is known about the fish assemblages, habitats, and ecology of rivers and reservoirs, there has been limited study of the fish assemblages in transitional habitats between these lotic and lentic habitats. Data about these river–reservoir interface (RRI) fish assemblages are needed to guide integrated management efforts of river–reservoir ecosystems. The aim of these efforts is to recommend flows for natural river function, conserve native riverine fish assemblages, and maintain reservoir sport fisheries. We used a multigear approach to assess the fish assemblages of four RRIs in the Colorado River Basin, Texas. In addition to characterizing RRI fish assemblages using species richness and evenness metrics, and habitat‐use guilds, we used a multivariate approach to evaluate intra‐annual shifts in species composition and abundance. All RRIs had high species richness and evenness values and included both macrohabitat generalist and fluvial species. RRIs also contained high proportions of the fish species available within each river–reservoir ecosystem, ranging from 55% to 80%. Observed intra‐annual shifts in RRI fish assemblages resulted from changes in abundance of dominant species rather than changes in species composition, with abundance of most species increasing from early spring to summer. Fish species responsible for intra‐annual shifts included mostly floodplain and migratory species, suggesting that species both used littoral habitats within RRIs and migrated through RRIs to river and reservoir habitats. The diversity of fishes found within RRIs highlights the importance of including these areas in future conservation and management efforts of river–reservoir ecosystems. Copyright © 2013 John Wiley & Sons, Ltd.     

RELATIONS BETWEEN ALTERED STREAMFLOW VARIABILITY AND FISH ASSEMBLAGES IN EASTERN USA STREAMS

Although altered streamflow has been implicated as a major factor affecting fish assemblages, understanding the extent of streamflow alteration has required quantifying attributes of the natural flow regime. We used predictive models to quantify deviation from expected natural streamflow variability for streams in the eastern USA. Sites with >25% change in mean daily streamflow variability compared with what would be expected in a minimally disturbed environment were defined as having altered streamflow variability, based on the 10th and 90th percentiles of the distribution of streamflow variability at 1279 hydrological reference sites. We also used predictive models to assess fish assemblage condition and native species loss based on the proportion of expected native fish species that were observed. Of the 97 sites, 49 (50.5%) were classified as altered with reduced streamflow variability, whereas no sites had increased streamflow variability. Reduced streamflow variability was related to a 35% loss in native fish species, on average, and a >50% loss of species with a preference for riffle habitats. Conditional probability analysis indicated that the probability of fish assemblage impairment increased as the severity of altered streamflow variability increased. Reservoir storage capacity and wastewater discharges were important predictors of reduced streamflow variability as revealed by random forest analysis. Management and conservation of streams will require careful consideration of natural streamflow variation and potential factors contributing to altered streamflow within the entire watershed to limit the loss of critical stream habitats and fish species uniquely adapted to live in those habitats. Published in 2011 by John Wiley & Sons, Ltd.     

SPATIAL PATTERNS OF BENTHIC INVERTEBRATES IN REGULATED AND NATURAL RIVERS

Daily fluctuating flows in regulated rivers can lead to extensive areas of substrate that experience drying and wetting. I investigated the longitudinal and lateral patterns of benthic invertebrates in the regulated peaking Magpie River and neighbouring natural rivers. Nearly half of all invertebrates collected in the Magpie River originated from the upstream reservoir. Both lentic and lotic invertebrates, however, decreased in abundance to natural levels 5–8 km downstream. Closest to the dam, lotic invertebrates were twice as abundant as those found in natural rivers. In natural rivers Diptera, Ephemeroptera, Coleoptera and Trichoptera were more common in the shallow and slower areas along transects. The opposite was true in the regulated Magpie where densities increased with water depth and velocity, particularly for the dominant groups Diptera and Trichoptera. The abundance of worm‐like organisms (e.g. Enchytraeidae, Lumbricidae, Naididae) and snails (Basommatophora) increased considerably in the varial zone. There were 10 times more Odonata and Plecoptera in the natural rivers, but lateral trends were not evident in either type of river. The influence of sampling location along transects on the interpretation of the community composition can be clearly seen in a resampling of the data. This study illustrates the presence of longitudinal and lateral gradients, and this knowledge needs to be incorporated into the design of research or monitoring programmes. Failing to do so could jeopardize decisions with the management of our flowing waters. Copyright © 2011 John Wiley & Sons, Ltd.     

CLASSIFICATION OF FLOW REGIMES FOR ENVIRONMENTAL FLOW ASSESSMENT IN REGULATED RIVERS: THE HUAI RIVER BASIN,CHINA

Flow regime characteristics (magnitude, frequency, duration, seasonal timing and rates of change) play a primary role in regulating the biodiversity and ecological processes in rivers. River classification provides the foundation for comparing the hydrologic regimes of rivers and development of hydro‐ecological relationships to inform environmental flow management and river restoration. This paper presents a classification of natural flow regimes and hydrologic changes due to dams and floodgates in the Huai River Basin, China, in preparation for an environmental flow assessment. The monthly natural flow regime of 45 stations in the upper and middle Huai River Basin were simulated for the period 1963–2000, based on the hydrological model SWAT (Soil and Water Assessment Tool). Six classes of flow patterns (low or high discharge, stable or variable, perennial or intermittent, predictable or unpredictable) were identified based on 80 hydrologic metrics, analysed by hierarchical clustering algorithms. The ecologically relevant climatic and geographic characteristics of these flow classes were tested for concordance with, and to strengthen, the hydro‐ecological classification. The regulation of natural flow patterns by dams and floodgates changed flows at some locations within each flow class and caused some gauges to shift into another class. The research reported here is expected to provide a foundation for development of hydro‐ecological relationships and environmental flow methods for wider use in China, as well as setting a new scientific direction for integrated river basin management in the Huai River Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

EFFECTS OF GRADE CONTROL STRUCTURES ON FISH PASSAGE,BIOLOGICAL ASSEMBLAGES AND HYDRAULIC ENVIRONMENTS IN WESTERN IOWA STREAMS: A MULTIDISCIPLINARY REVIEW

Land use changes and channelization of streams in the deep loess region of western Iowa have led to stream channel incision, altered flow regimes, increased sediment inputs, decreased habitat diversity and reduced lateral connectivity of streams and floodplains. Grade control structures (GCSs) are built in streams to prevent further erosion, protect infrastructure and reduce sediment loads. However, GCS can have a detrimental impact on fisheries and biological communities. We review three complementary biological and hydraulic studies on the effects of GCS in these streams. GCS with steep (≥1:4 rise : run) downstream slopes severely limited fish passage, but GCS with gentle slopes (≤1:15) allowed greater passage. Fish assemblages were dominated by species tolerant of degradation, and Index of Biotic Integrity (IBI) scores were indicative of fair or poor biotic integrity. More than 50% of fish species had truncated distributions. After modification of GCS to reduce slopes and permit increased passage, IBI scores increased and several species were detected further upstream than before modification. Total macroinvertebrate density, biomass and taxonomic diversity and abundance of ecologically sensitive taxa were greater at GCS than in reaches immediately upstream, downstream or ≥1 km from GCS. A hydraulic study confirmed results from fish passage studies; minimum depths and maximum current velocities at GCS with gentle slopes (≤1:15) were more likely to meet minimum criteria for catfish passage than GCS with steeper slopes. Multidisciplinary approaches such as ours will increase understanding of GCS‐associated factors influencing fish passage, biological assemblage structure and other ecological relationships in streams. Copyright © 2011 John Wiley & Sons, Ltd.     

EFFECTS OF FLOW RELEASES ON MACROINVERTEBRATE ASSEMBLAGES IN THE INDIAN AND HUDSON RIVERS IN THE ADIRONDACK MOUNTAINS OF NORTHERN NEW YORK

The effects of flow releases (daily during spring and four times weekly during summer) from a small impoundment on macroinvertebrate assemblages in the lower Indian River and upper Hudson River of northern New York were assessed during the summers of 2005 and 2006. Community indices, feeding guilds, dominant species and Bray–Curtis similarities at three sites on the Indian River, below a regulated impoundment, were compared with those at four control sites on the Cedar River, below a run‐of‐the‐river impoundment of comparable size. The same indices at four less‐likely affected sites on the Hudson River, below the mouth of the Indian River, were compared with those at an upstream control site on the Hudson River. Results show that the function and apparent health of macroinvertebrate communities were generally unaffected by atypical flow regimes and/or altered water quality at study reaches downstream from both dams in the Indian, Cedar and Hudson Rivers. The lentic nature of releases from both impoundments, however, produced significant changes in the structure of assemblages at Indian and Cedar River sites immediately downstream from both dams, moderate effects at two Indian River sites 2.4 and 4.0 km downstream from its dam, little or no effect at three Cedar River sites 7.2–34.2 km downstream from its dam, and no effect at any Hudson River site. Bray–Curtis similarities indicate that assemblages did not differ significantly among sites within similar impact categories. The paucity of scrapers at all Indian River sites, and the predominance of filter‐feeding Simulium gouldingi and Pisidium compressum immediately below Abanakee dam, show that only minor differences in dominant species and trophic structure of macroinvertebrate communities occurred at affected sites in the Indian River compared to the Cedar River. Thus, flow releases had only a small, localized effect on macroinvertebrate communities in the Indian River. Copyright © 2011 John Wiley & Sons, Ltd.     

MOMENTUM,ENERGY AND DRAG COEFFICIENTS FOR ICE‐COVERED RIVERS

A lack of reliable hydraulic parameters has been a main factor hindering the progress in predicting ice‐covered river flows; the predictions need input hydraulic parameters such as the energy, momentum and drag coefficients (α, β and C_D). In this paper, a large volume of winter measurements of flow velocity collected from 26 ice‐covered rivers is analysed to determine the coefficients. Using cross‐sectionally distributed streamwise velocities, α and β are evaluated directly. They are also derived from empirical relationships. For both the riverbed and ice cover, C_D is evaluated on the basis of turbulent boundary‐layer theory and the quadratic law for friction. The results show that ice‐covered river flows feature a number of velocity distributions: a single core of high velocities in the thalweg, a single core of high velocities off the thalweg and multiple cores of relatively high velocities at the cross section. The velocity distributions are significantly non‐uniform. Direct evaluations give overall averages of α = 1.23 and β = 1.08. They represent 22% and 8.3% corrections to the literature values (overestimates). An examination of the velocity distributions reveals that the ratio of the maximum velocity to the cross‐sectionally averaged velocity equals 1.356. It is recommended that values of C_D = 0.004 ± 0.0005 and 0.002 ± 0.0005 be used for the riverbed and ice, respectively. This paper discusses turbulence shear stress and the associated length scale in the boundary layer as well as winter discharges. The results have applications to aquatic ecology, water resources development and flood prevention. Copyright © 2012 John Wiley & Sons, Ltd.     

Short‐term effects in a reduced flow stretch: The case of the Antas River in South Brazil

The building of adduction channels (penstocks) that conduct water from reservoirs to turbines, which are located kilometres from the dam, is becoming common, optimizing the electricity generation in small dams. This design creates a river stretch with reduced discharge between the dam and the powerhouse. This study evaluates the short‐term impacts of the below‐dam decrease in river flow on fish assemblages. Samples were collected in the reduced flow stretch of the Castro Alves Hydropower Plant (Antas River, Rio Grande do Sul, Brazil) before the reservoir started operating (January 2008; mean discharge of 103.7 m³/s) and immediately after operation began (March 2008; mean discharge of 12.4 m³/s). Sampling was conducted in distinct habitats of the reduced flow stretch (slow waters—gillnets, sand beaches—seining nets, structured littoral—electrofishing, and fast waters—cast nets) with a strongly standardized effort. The attributes of the fish assemblages were not negatively affected by the flow reduction in any habitat sampled. However, distinct changes in the spatial structure were observed considering the different types of habitat predominantly used by the species, which represents an entire reorganization of the fish assemblages in the short term. It is fundamental that these short‐term aspects be considered in the licensing of hydropower plants in addition to the long‐term changes.     

USING AN EXPERIMENTAL IN SITU FISHWAY TO PROVIDE KEY DESIGN CRITERIA FOR LATERAL FISH PASSAGE IN TROPICAL RIVERS: A CASE STUDY FROM THE MEKONG RIVER,CENTRAL LAO PDR

Fish passage through an experimental vertical‐slot fishway was assessed at a floodplain regulator on the Mekong River in Central Laos between April and July 2009. Experiments were conducted to investigate the influence of fishway floor slope (1v:15h or 1v:7.5h) on fish passage success with a view to developing a series of optimal design criteria for the construction of vertical‐slot fishways at other barriers to fish passage in the Lower Mekong Basin. A total of 14 661 fish from 73 species were captured during the experiments. Catches were dominated by riverine (white) (n = 51; 69% of total) and floodplain (black) species (n = 15; 20%) which represented 19 families in total. The work demonstrated that fish were actively attempting upstream passage from the Mekong River to an adjacent floodplain and displayed strong migratory behaviour during river level rises. Migratory activity was greatest during sharp rises in water level but reduced substantially when river level fell. Fish community composition varied greatly among the two fishway floor slopes and the control group. More fish species were collected from control samples, but the most fish and species were collected when the fishway was configured on a moderate hydraulic slope (1v:15h). A range of size classes were also collected from control and moderate‐slope groups, but steeper‐gradient catches were dominated by larger fish. This study demonstrated that vertical‐slot fishways could provide passage for a biodiverse fish community where fish move laterally onto floodplains. The construction of fishways which consider the local fish ecology and hydrology may therefore represent a valuable management tool to help restore important movement pathways for tropical freshwater fish. Copyright © 2011 John Wiley & Sons, Ltd.     

NATURAL REGENERATION OF FISH ASSEMBLAGES IN THE PILICA RIVER AFTER A REDUCTION OF POINT‐SOURCE POLLUTION

Electrofishing was conducted along the Pilica River, the biggest West tributary of the Vistula, Poland. This paper presents a comparison between the 1990s (1994–1995, 63 sites) and 2000s (2003–2005, 64 sites), that is, because the water quality started improving after 1989. In the Pilica, there is an increased risk of masking relationships between fish distribution and abiotic factors because a dam without a fish pass was constructed in the middle course of the river in 1973, perhaps resulting in fewer possibilities for potential colonizers reaching river fragments fit for colonization. The main aim of this study was to check if natural fish fauna regeneration took place in the river following a significant sewage input reduction. The fish samples were classified with a Kohonen artificial neural network and assigned to two main clusters (X and Y) and, respectively, two pairs of subclusters (X₁, X₂; Y₁, Y₂) of neurons. No species attained its significantly highest biomass and/or frequency (expressed as IndVal) in X₂ (in which samples from the 1990s dominated), whereas 21 species did so in Y₁ or Y₂ (containing almost all of the samples from the 2000s), better testifying to the aquatic environment at the sites where the samples assigned to two latter subclusters come from. Moreover, significantly higher values for Y₁ and/or Y₂ when compared with X₂ were recorded in the richness of the fish species, the total fish biomass and total rheophilic fish biomass. In summary, the positive reaction of ichthyofauna to the improvement of the water quality, stated after 1989, was not recorded during the sampling in 1994–1995, but with a considerable delay in 2003–2005. It should be emphasized that the improvement in fish assemblages was recorded in the Pilica both upstream and downstream from the dam reservoir. A good ecological status was recorded even for some samples from the 1990s. They were collected in the lower river course, where the fish could move freely from the Vistula. This testifies the combined role of the presence of potential colonizers and the functioning ecological corridors for fish assemblages in a river system under human impact. Copyright © 2012 John Wiley & Sons, Ltd.     

EVALUATION OF SPAWNING SUBSTRATE ENHANCEMENT FOR WHITE STURGEON IN A REGULATED RIVER: EFFECTS ON LARVAL RETENTION AND DISPERSAL

Site‐specific habitat alterations have improved spawning success and early life stage survival of different fish species, including sturgeon, in regulated rivers. We modified the substrate within a section of river at the only known spawning site used by white sturgeon (Acipenser transmontanus) in the Mid Columbia River, Canada. Existing armoured riverbed conditions were modified using a mixture of larger and smaller angular rock with the assumption that the larger material would remain in place at higher discharges and help retain the smaller material. This increased substrate complexity and the amount of available interstitial spaces. We stocked 2‐day posthatch larvae over both the modified site and at an adjacent control site that represented existing substrate conditions. Our objectives were to determine (i) the extent that stocked larvae remained in both the modified and control sites immediately after release, (ii) the timing of subsequent dispersal of larvae from both sites and (iii) how total length of dispersing larvae changed over time and by site. Results from this work indicated that the modified section of riverbed retained significantly higher numbers of larvae after release compared with the control site. Larvae at the modified site were able to hide and remain within the substrate and initiated downstream drift 15 days after release. With the exception of the first day after release, dispersal from both sites occurred at night. There was a significant effect of time after release and site on the total length of dispersing larvae. The larger variation in total larval length observed at the control site compared with the modified site indicated greater difficulty in hiding within the control substrate. Larvae initiated dispersal from the modified site at a mean size of 17.5 mm, which may indicate an important growth threshold before drift. Results from this work are important for future mitigative efforts for sturgeon in regulated rivers where changes to spawning substrates have occurred. Copyright © 2012 John Wiley & Sons, Ltd.     

EVALUATION OF THE SOUNDING ROD METHOD FOR SAMPLING COARSE RIVERBED SEDIMENTS IN NON‐WADEABLE STREAMS AND RIVERS

The substrate of fluvial systems is regularly characterized as part of a larger physical habitat assessment. These measures are instrumental in meeting the regulatory responsibilities of bioassessment and monitoring programs, and essential to monitoring restoration and rehabilitation success. We describe and validate a commonly used technique for broadly categorizing, and thus characterizing, the substrate in non‐wadeable streams and rivers called the sounding rod method. In brief, a rod, often hollow, is used to probe the substrate of non‐wadeable systems to characterize the substrate. We tested the viability of this method on three different systems by comparing estimated particle class and direct particle measurements. Our results indicate that substrates can adequately be defined into six broad classes (fine‐particle sediment, sand, gravel, cobble, boulder and bedrock) based on size using the sounding rod. Estimated classes were significantly positively correlated to measured classes (τ = 0.83, p < 0.001), and estimates of size class and direct measurements of size were not from significantly different distributions (χ² _0.05,9 = 569.51, p < 0.001). Further, there were significant differences between each category (H = 243.5, 3 d.f., p < 0.001). Although our results affirm that actual substrate class size can be directly inferred from estimated data, it should be noted that soft sediments <200 mm were not always detected. This finding should be carefully considered with individual study objective. Overall, the sounding rod method can be learned quickly, and it is a low‐cost and time‐efficient method for substrate classification. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

PROJECTED FLOW ALTERATION AND ECOLOGICAL RISK FOR PAN‐EUROPEAN RIVERS

Projection of future changes in river flow regimes and their impact on river ecosystem health is a major research challenge. This paper assesses the implications of projected future shifts in river flows on in‐stream and riparian ecosystems at the pan‐European scale by developing a new methodology to quantify ecological risk due to flow alteration (ERFA). The river network was modelled as 33 668 cells (5′ longitude × 5′ latitude). For each cell, modelled monthly flows were generated for an ensemble of 10 scenarios for the 2050s and for the study baseline (naturalized flows for 1961–1990). These future scenarios consist of combinations of two climate scenarios and four socio‐economic water‐use scenarios (with a main driver of economy, policy, security or sustainability). Environmental flow implications are assessed using the new ERFA methodology, based on a set of monthly flow regime indicators (MFRIs). Differences in MFRIs between scenarios and baseline are calculated to derive ERFA classes (no, low, medium and high risk), which are based on the number of indicators significantly different from the baseline. ERFA classes are presented as colour‐coded pan‐European maps. Results are consistent between scenarios and show that European river ecosystems are under significant threat with about two‐thirds at medium or high risk of change. Four main zones were identified (from highest to lowest risk severity): (i) Mediterranean rim, southwest part of Eastern Europe and Western Asia; (ii) Northern Europe and northeast part of Eastern Europe; (iii) Western and Eastern Europe; and (iv) inland North Africa. Patterns of flow alteration risk are driven by climate‐induced change, with socio‐economics as a secondary factor. These flow alterations could be manifested as changes to species and communities, and loss of current ecosystem functions and services. Copyright © 2013 John Wiley & Sons, Ltd.     

HISTORIC CHANGES (1941–2008) IN SIDE CHANNEL AND BACKWATER HABITATS ON AN UNCHANNELIZED REACH OF THE MISSOURI RIVER

Flow regulation has had pervasive effects on aquatic ecosystems within the world's large rivers. While channelization on the lower Missouri River has led to major changes in the river and its floodplain, including the loss of shallow water habitats, effects of upstream dams on unchannelized reaches on the Missouri have not been formally assessed. We quantified changes in the number and size of off‐channel habitats, specifically backwaters and side channels, on the 95‐km unchannelized reach of the Missouri below Gavins Point Dam (Yankton, South Dakota) using historical (1941, 1983–1985, 2008) aerial imagery. Total and mean areas of side channels declined by 77% and 37% and total and mean length decreased by 79% and 42% from 1941 to 2008. Total area of backwaters increased by 40% from 1941 to 2008, whereas mean area decreased by 36%. Our findings suggest that sharp declines in the area and length of side channels have occurred on this unchannelized remnant reach of the Missouri River, with likely significant impacts on aquatic ecosystem processes. Copyright © 2011 John Wiley & Sons, Ltd.     

DYNAMICS OF IN‐STREAM WOOD AND ITS IMPORTANCE AS FISH HABITAT IN A LARGE TROPICAL FLOODPLAIN RIVER

The recruitment of wood from the riparian zone to rivers and streams provides a complex habitat for aquatic organisms and can influence both aquatic biodiversity and ecosystem function. The Daly River in the wet–dry tropics of northern Australia is a highly seasonal, perennially flowing sand‐bed river where surveys of river wood aggregations at the reach scale (~2 km) in 2008 and 2009 recorded densities of 37–78 km^?1 and identified distinct types of river wood aggregations: key pieces, standing trees, fallen trees, wrack and single pieces. After larger than average flows in the 2008/2009 wet season, between 46% and 51% of the surveyed river wood had moved. The distribution of wood age classes indicated continual recruitment and slow turnover of wood within the river. Surveys of fish and habitat characteristics at the mesohabitat scale (~100 m) showed fish species richness; diversity and fish abundance were not correlated to the proportion of wood present. Fish assemblage structure was associated with wood cover as well as other environmental variables such as stream width and depth. The importance of in‐stream wood also varied for different species and age classes of fish. This study documents the dynamic nature of river wood aggregations and their complex and variable distribution and suggests their importance as fish habitat in this tropical river. Copyright © 2012 John Wiley & Sons, Ltd.     

ASSESSING THE IMPACTS OF RIVER REGULATION ON NATIVE BULL TROUT (SALVELINUS CONFLUENTUS) AND WESTSLOPE CUTTHROAT TROUT (ONCORHYNCHUS CLARKII LEWISI) HABITATS IN THE UPPER FLATHEAD RIVER,MONTANA, USA

Hungry Horse Dam on the South Fork Flathead River, Montana, USA, has modified the natural flow regimen for power generation, flood risk management and flow augmentation for anadromous fish recovery in the Columbia River. Concern over the detrimental effects of dam operations on native resident fishes prompted research to quantify the impacts of alternative flow management strategies on threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi) habitats. Seasonal and life‐stage specific habitat suitability criteria were combined with a two‐dimensional hydrodynamic habitat model to assess discharge effects on usable habitats. Telemetry data used to construct seasonal habitat suitability curves revealed that subadult (fish that emigrated from natal streams to the river system) bull trout move to shallow, low‐velocity shoreline areas at night, which are most sensitive to flow fluctuations. Habitat time series analyses comparing the natural flow regimen (predam, 1929–1952) with five postdam flow management strategies (1953–2008) show that the natural flow conditions optimize the critical bull trout habitats and that the current strategy best resembles the natural flow conditions of all postdam periods. Late summer flow augmentation for anadromous fish recovery, however, produces higher discharges than predam conditions, which reduces the availability of usable habitat during this critical growing season. Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances of protecting key ecosystem processes and help to maintain and restore threatened bull trout and westslope cutthroat trout populations in the upper Columbia River Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

SHIP‐LOCK–INDUCED SURGES IN AN IMPOUNDED RIVER AND THEIR IMPACT ON SUBDAILY FLOW VELOCITY VARIATION

The majority of large rivers are fragmented by dams, and navigation is often supported by the installation of ship locks. Despite their ubiquitous existence, the effect of ship locks on river basin hydrodynamics is rarely considered in an environmental context. Ship‐lock operation induces single‐wave crests or troughs called surges, which propagate along the basin and are subject to reflection at the up‐ and downstream impoundments. We used pressure sensors and acoustic Doppler current profiler measurements to investigate the effects of up‐ and downstream ship locking on the water level and the current velocity dynamics in a 12.9‐km‐long basin of the impounded river Saar (Germany). Ship lifting at the lower dam and the associated water export from the basin results in a negative surge propagating upstream, whereas a descending ship‐lock operation at the upper dam of the basin creates a positive surge propagating downstream. Both types of waves are subject to positive reflection at the opposing dams. Frequent lock operations lead to a complex pattern of multiple superimposing surges. The resulting water level fluctuations are comparable in magnitude with those associated with discharge variations due to hydropower peaking but occur at much shorter timescales. Associated with the water surface displacement during wave passage is a corresponding increase or decrease of the longitudinal current velocity. The magnitude of wave‐induced velocity fluctuations can exceed mean flow velocities by a factor of three and, depending on wave type and direction, can result in a reversal of the main flow direction of the river. Because of their longevity of several hours and superposition effects, ship‐lock–induced surges govern 66.3% and 45.4% of the subdaily variations in flow velocity and water level, respectively. This article concludes with a discussion of the potential effects of lock‐induced flow dynamics in impoundments on oxygen dynamics and methane ebullition. Copyright © 2013 John Wiley & Sons, Ltd.